Show feedback in the admin panel and as aggregate stats on the briefing page

Feedback was only ever written to session_setting rows with no way to
view it short of querying the database directly. Adds a FeedbackService
that pulls per-player feedback entries and aggregate averages, backing
two new views:

- /admin/feedback: a full table of every submission (game, session,
  player, ratings, comment) plus summary tiles, linked from the admin
  sidebar.
- /briefing: a "Field Reports" block with the average difficulty/
  entertainment/theme ratings, shown to prospective players. Free-text
  comments are deliberately left off this public page since they're
  unmoderated player input.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/game/admin/base.html.twig

@@ -101,6 +101,7 @@
                 { route: 'game_admin_users',     label: 'Users',      icon: '👤' },
                 { route: 'game_admin_games',     label: 'Games',      icon: '🎮' },
                 { route: 'game_admin_sessions',  label: 'Sessions',   icon: '▶' },
+                { route: 'game_admin_feedback',  label: 'Feedback',   icon: '💬' },
                 { route: 'game_admin_email_log', label: 'Email Log',  icon: '✉' },
             ] %}